The present invention relates to a wide area mobile communications system and, more particularly, to a mobile communications system which allows any remote mobile stations to hold communication with each other through a public switching network.
A cordless telephone has been known which is a radio telephone unit connecting to a subscriber's line. The problem with the cordless telephone is that it does not work when brought out of a narrow predetermined service area and, outside the service area, its role has to be fulfilled by alternative means such as a public telephone. While a wide area radio communications service may be implemented by a paging receiver or pager, the function assigned to a pager is only receiving a call and not originating a call; for originating a call, a public telephone or like means has to be relied on.
One possible approach for a wide area telephone service is a hybrid system which employs a cordless telephone for originating a call and a pager for recognizing receipt of a call. Specifically, a transmission base station which governs a wide area calls up a mobile station through a pager, while the mobile station originates a call for any of fixed stations distributed in a plurality of narrow areas. An example of such a hybrid system is shown in FIG. 1. The system in FIG. 1 includes a public switching network 10, a subscriber's line 12, a public telephone line 14, and a transmission base station 16. The operation of the system will be described assuming a case wherein a mobile station 22 operable in a certain specific area S.sub.1 is to call up another mobile station 18 which is connected by radio to a fixed station 20 which in turn is connected to the public telephone line 14 or the like.
The mobile station 22 starts the communication with connecting itself to a fixed station 24 over a radio channel to thereby transmit a signal for calling the mobile station 18 to the exchange 10 over the subscriber's line 12. Then, the exchange 10 sends a page instruction to the base station 16 so as to cause it to transmit a call signal assigned to the mobile station 18. When the mobile station 18 is called by the call signal from the base station 16, a person at the mobile station 18 originates a call for the calling mobile station. The call signal from the mobile station 18 is received by the fixed station 20 which is allocated to the specific area S.sub.2. Then, the fixed station 20 is connected to the fixed station 24 via the exchange 10, whereby communication is set up between the mobile stations 18 and 22.
However, a problem arises when both the mobile stations to originate a call and receive a call are of the kind which is connectable to a public telephone line or the like. Assume a case wherein, as shown in FIG. 2, a mobile station 34 located in an area S.sub.3 governed by a fixed station 32 which is connected to a public telephone line 28 intends to call up a mobile station 38 located in an area S.sub.4 governed by a fixed station 36 which is also connected to a public telephone line 30. The procedure from the origination of a call at the mobile station 34 to the receipt of a call at the mobile station 38 is the same as in FIG. 1. However, despite the call meant for the mobile station 34, it is routed to the mobile station 34 by way of the exchange 10 and then the base station 16 by a radio wave 40, preventing the connection from being established for ever.